1. Field of the Invention
The present invention relates to a nut runner control system and a method of monitoring nut runners, and more particularly to a nut runner control system and a method of monitoring nut runners such that drive conditions for the nut runners are set up and modified by a master controller, the nut runners are controlled through subcontrollers, respectively, and operating conditions of the nut runners are monitored by the master controller through the subcontrollers.
2. Description of Related Art
The assembly process for various industrial products such as automobiles is more and more automated to meet increased efficiency and labor saving requirements. For example, some threaded parts such as bolts, nuts, or the like are automatically fastened by a system of nut runners.
Japanese Laid-Open Patent Publication No. 58(1983)-66672 discloses a system comprising a plurality of nut runners for tightening threaded parts. In order to prevent the fastening torques of the nut runners from varying from each other, torque preset means for the respective nut runners preset preliminary torques smaller than desired tightening torques, and when the tightening torques have reached the preliminary torques, the nut runners are temporarily stopped, and then the nut runners are operated again to tighten the threaded parts with the desired tightening torques after elapse of a certain period of time.
If the preset preliminary torques are to be changed, however, they have to be modified for the respective nut runners. It has been quite tedious and time-consuming to preset and modify the drive conditions for the respective nut runners.
In the nut runner control system, it is desirable to monitor the conditions of how the threaded parts are tightened by the nut runners and how the nut runners are operating, so that any trouble will be prevented from occurring and repairs will quickly be carried out should a problem happen.
For example, Japanese Laid-Open Patent Publication No. 55(1980)-18349 shows a system in which the period of time from the start of a tightening process of a nut runner to the time at which the tightening torque has reached a prescribed torque is measured to determine whether the threaded part has been successfully tightened or not. Japanese Laid-Open Patent Publication No. 59(1984)-118380 discloses a system in which the rotational angle and tightening torque of a nut runner are detected and compared with preset angle and torque data to determine whether the tightening process is successful or not.
The process of determining whether a threaded part has been tightened as desired in these disclosed systems works well only when the nut runner and its associated devices operate normally, but cannot determine any tightening failures resulting from malfunctions of the nut runner itself and its associated devices. Therefore, when such a tightening failure happens, the cause of the tightening failure cannot be found. As a result, a number of nut runners cannot efficiently be operated in a production line based on the conventional control system.